Semiconductor processes typically include such operations as growing or depositing oxides on wafer surfaces and diffusing impurities such as conductivity type determining dopants into the semiconductor material of the wafers. During such processes semiconductor wafers are heated in furnaces to temperatures which typically range between 700.degree. and 1250.degree. C. At such elevated temperatures thermal migration of atoms proceeds at a sufficient rate to permit such dopants to be diffused into preselected regions of the wafers and form desired functional devices in such wafers.
Semiconductor device characteristics are predictably controlled by the type and concentration of the diffused dopants in such regions of a particular device. Typically, a preselected time-temperature profile in a controlled environment of a furnace brings about a predictable result. However, problems occur in the control of the environment within the diffusion furnaces in that certain harmful impurities tend to be introduced into the furnaces while the furnaces operate at such elevated temperatures. Adverse effects of such harmful impurities show a tendency to increase with time.
A typical prior art furnace is in essence a concentric structure of an inner process tube, a liner, resistance heating elements of, for example, a metallic alloy such as Kanthal, and an outer, insulating support structure. The inner process tube encloses and defines a process chamber into which the wafers are placed. Quartz and silicon are known to possess sufficient structural integrity for a process tube at the temperatures to which the tube is typically subjected.
The liner, of a ceramic material, is typically interposed between the heating elements and the process tube to serve as a guide for centering the process tube and to distribute heat emanating from the heating elements. The heating elements in some furnaces are also supported by such a ceramic structure to add rigidity to the heating elements at these elevated temperatures. The outermost support structure holds the ceramic backup structure in place, to permit a concentric mounting of the process tube, and provides insulation against heat loss from the furnace.
Experience has shown that the process tube in such a prior art furnace needs to be replaced from time to time in an effort to purge the environment within the process chamber of harmful impurities. It appears that as the diffusion furnace is operated, the concentration of the harmful impurities gradually increases in the wall of the process tube and such impurities evaporate from the wall to the wafers which are being processed within the tube. Generally, such harmful impurities are impurity atoms with an energy level between, and most effectively about halfway between the valence and the conduction band of the semiconductor material. The presence of such impurities in the semiconductor base material provides unwanted generation or recombination centers for minority carriers. In many semiconductor device structures the presence of such generation or recombination centers alters the device characteristics in an undesired and frequently unforeseen manner.
In an initial burn-in period of a newly installed process tube an acceptably low level of diffusing harmful impurities becomes established in such tube of a diffusion furnace. During subsequent operations of the furnace the presence of the harmful impurities in the process tube tends to increase with an accompanying diffusion of such impurities into wafers which are processed within the tube. Ultimately, the harmful impurities reach a contamination level at which the process tube is replaced by an unused tube. It is, of course, possible to reduce unwanted characteristics caused by the harmful impurities through known gettering techniques. However, it is more desirable to eliminate the harmful impurities before they reach the wafers.
It also has been noted that the quartz material typically used in the process tubes has a tendency to devitrify. Such a devitrification is not apparent when silicon is used as a material for the process tubes. The devitrification of the quartz tends to shorten the useful life of the process tube, and in some cases it is possible that the tube may have to be replaced even before contamination by harmful impurities reaches an unacceptable level.
However, when a silicon process tube is used in the furnace, contamination by harmful impurities reaches unacceptable levels sooner than when a quartz tube is used under substantially similar conditions.
It is consequently desirable to provide a diffusion furnace with a structure having an increased life cycle over prior art furnaces.